Within the field of packaging technology and engineering, use is often made of single-use disposable packages or cartons produced from plastic-coated packaging material for packing and transporting liquid foods such as milk, juice, wine etc. The exact structure of such a packaging material varies depending upon what product is to be packed, but generally includes one or more body or carrier layers of paper or cardboard which impart to the package the requisite rigidity and configurational stability, and outer coatings of a thermoplastic, preferably polythene, which not only make the package liquid-tight but also make it possible to give the package its permanent desired geometric final form by means of so-called heat sealing.
Nowadays, disposable packages of the type contemplated here are most generally produced by means of modern, rational packaging machines which, either from a web or from prefabricated blanks of the packaging material, forms, fills and seals finished packages. For example from a web, packages of the Tetra Brik.RTM. type are produced in that the Web is first formed into a tube by the longitudinal edges of the web being united to one another in a longitudinal overlap seal or splice. The tube is filled with the desired contents and divided into closed package units by repeated transverse seals of the tube along mutually spaced-apart transverse tube zones beneath the level of the contents of the tube. The particular cushion-like packages are thereafter given their desired geometic final form (in this specific case parallel-epipedic) by an additional fold forming and sealing operation during which four double-walled triangular tabs or flaps occur at the corners of the package. Of these flaps, both of the upper corner flaps of the package are folded down and sealed to adjacent vertical side walls, while both of the lower corner flaps of the package are folded in and sealed against the planar bottom wall of the package.
The described prior art parallelepipedic package thus displays two sealing fins occasioned by transverse sealing of the tube, in which fins opposing material layers of the packaging material are united to one another inside-to-inside in a sealing splice formed by surface fusion. One of these two sealing fins is, in such instance, located at the planar upper wall of the package, transversely over which it extends from tip to tip of both of the upper corner flaps folded down towards the side walls of the package and sealed. The other sealing fin is correspondingly located at the bottom wall of the package across which it extends from tip to tip of both of the lower corner flaps folded in towards the bottom wall of the package and sealed. Moreover, the package has a longitudinal sealing splice which constitutes a part of the longitudinal overlap splice of the produced tube and which intersects the above-mentioned transverse sealing fins at a point between both of the corner flaps at the upper and lower end walls of the package, respectively.
The demands placed on a disposable package of the type described here are, of course, that it must provide as good protection as possible to the packed product, but also that it must be easy to handle during both transport and use. In order to make the package easy to handle when using, it is often provided with some type of opening arrangement with the aid of which the contents of the package must be made easily accessible and easy to pour from the package in a single, well-directed stream.
In the above-described package of the Tetra Brik.RTM. type, use is here most generally made of one of the two upper triangular corner tabs or flaps of the package as the pour aperture or spout, the package proper being normally opened in that this corner flap is released and raised from its position folded down towards the side wall of the package and that the sealing fin extending over the corner flap is torn or cut off so that a communication channel is established with the interior of the package.
Since a certain degree of difficulty may be experienced in tearing the above-mentioned triangular flap along a prepared tear indication or perforation, and since cutting off the sealing fin presupposes access to some form of utensil, a solution to the problem of opening such packages has instead been proposed in the art, using a tear thread or tear strip of the type described, for example, in Swedish Patent Specification 402,899. In such instance, the tear thread or tear strip is inlaid in the sealing fin from the tip of the triangular corner flap which serves as the pouring aperture or spout, up to and past the point of intersection of the sealing fin with the longitudinal overlap splice of the package so that the tear thread or tear strip becomes accessible from the outside of the package. When the package is to be opened, the exteriorly accessible portion of the tear thread or tear strip is drawn upwardly and rearwardly, whereby the sealing splice in the sealing fin between the above-mentioned point of intersection and the tip of the triangular corner flap is severed, for the formation of a corresponding pour aperture.
However, this prior art thread- or strip-assisted opening form of package has proved to function less satisfactorily, primarily because the thread or strip seldom cuts directly in the sealing splice but instead cuts between the paper material and the plastic coating on one of the material layers. As a result the paper or cardboard layer of the packaging material is exposed so that the contents of the package come into contact with the absorbent, exposed fiber surface which rapidly becomes soggy and loses rigidity.
Another prior art solution to the problem of opening packages of the type comtemplated here is described in, for example, European Patent Application carrying Publication Number 0083441. In this case, the opening arrangement consists of a strip of laminated material which is inlaid in the sealing fin and comprises partly an inner layer of a material of high tensile strength and tearing resistance, eg. oriented polyester, nylon, metal foil, etc., and partly outer coatings of a heat sealable plastic material, preferably polythene, disposed on either side of the inner layer. In such instance, the opening strip extends, like the above-described tear thread or tear strip, from the tip of the corner flap serving as the pouring aperture or spout, up to and past the point of intersection between the sealing fin and the longitudinal overlap splice of the package and is, by means of the outer thermoplastic coatings of the opening strip, fixedly sealed to the opposing thermoplastic coatings of the package within the region of that portion of the sealing fin accommodating the strip. Since, in sealing terms, the material in the inner layer of the opening strip is hardly compatible, or totally incompatible, with the outer thermoplastic coatings of the opening strip, the inner layer of the opening strip is subjected to some form of adhesion-improving pretreatment, so-called priming, and/or a suitable sealing temperature is selected, to achieve the desired sealing strength or bond between the inner layer of the opening strip and its outer thermoplastic coatings. The demand placed in this instance is that the sealing strength between the inner layer of the opening strip and its outer thermoplastic coatings must be weaker than the sealing strength between the outer thermoplastic coatings of the opening strip and the thermoplastic coatings of the packaging material proper, while at the same time the sealing strength must be sufficiently great to prevent spontaneous delamination of the opening strip when the package is subjected to external stresses during normal transport and handling.
The opening procedure in a package with a laminated opening strip of the above-described type is based on the concept that the opening strip, on tearing of the sealing fin, is to delaminate in the interface layer between the inner layer and the outer thermoplastic coatings, for exposing the inner layer which is pulled off, at the same time as both of the outer thermoplastic coatings are left adhered against the packaging material in the region of the pouring aperture created. The problem in this prior art form of opening strip is that, in practice, it has proved difficult to optimize the adhesion strength between the inner layer of the opening strip and its outer thermoplastic coatings, and, in order to avoid an excessively low adhesive strength and consequentiaI risk of spontaneous or unintentional delamination of the opening strip, the adhesion pretreatment or priming of the inner layer of the opening strip and/or the sealing temperature governing the adhesion strength are often intentionally selected such that an excessively great binding strength between the inner layer and the outer thermoplastic coatings becomes inevitable. This in turn entails that the opening strip becomes difficult to pull off without applying extreme tensile forces and/or that pulling-off of the opening strip not uncommonly results in the inner thermoplastic coatings of the packaging material proper also being pulled-off, whereby the fiber layers of the packaging material are exposed within the region of the pouring aperture or spout.
Swedish Patent Specification 459,916 describes another prior art opening principle using a laminated opening strip which, in analogy with the previously described opening strip, includes an inner layer of a material of good tensile strength and tearing resistance, preferably polyester, and outer coatings of sealable plastic material, preferably polythene. In such instance, the adhesion strength between all of the material layers included in the opening strip is intentionally high, with a view to making possible pulling-off of the opening strip Without delamination between the layers in the opening strip. Instead, easy pull-off of the opening strip is ensured without requiring the employment of excessive physical force in that the packaging material, in that portion of the sealing fin accommodating the strip, has been provided with a seal-weakening agent, eg. water glass, localized between the fiber layers of the packaging material and its inner thermoplastic coating, whereby the sealing strength between the packaging material layers is reduced in such a manner that the bonding strength between the outer thermoplastic coatings of the opening strip and the inner thermoplastic coatings of the packaging material is higher than the bonding strength between the fiber layers of the packaging material and its inner thermoplastic coating. Thus, on pulling-off of the opening strip, the plastic coatings within the seal-weakened region of the sealing fin will also be torn off and accompany the opening strip, at the same time as the fiber layer at least partly impregnated with the water glass is exposed within the region of the pouring aperture. In order to function well, this opening arrangement presupposes that at least the major portion of the thus exposed fiber surface is impregnated with the moisture and liquid-protective water glass film, but in practice it has proved difficult to satisfy this requirement and it is not uncommon that the fiber surface, after tearing-off of the opening strip, is left without adequate protection.